The present invention relates to the field of audio processing and in particular to an apparatus and a method for the noise-dependent adaptation of an acoustic useful signal at a listening location.
When audio is played in a noisy environment, listening pleasure is often impaired by background noise. For example, the interior of a motor vehicle is a noise-filled space in which music and speech is frequently listened to. The background noise can originate from the engine, tires, ventilation, and other units in the motor vehicle. In addition, the noise may depend on the speed, the road conditions, and the operating conditions of the motor vehicle. A passenger in the motor vehicle generally responds to this constantly changing noise pattern by manually adjusting the volume of the audio being listened to.
A technique for dynamic sound optimization is disclosed in U.S. Pat. No. 5,434,922 entitled xe2x80x9cMethod and Apparatus for Dynamic Sound Optimizationxe2x80x9d. This patent discloses that the volume of the acoustic useful signal is automatically adjusted as a function of the background noise. A microphone captures the total sound in the listening room, and the total signal (i.e., including the desired signal and noise) is processed to extract the noise component from the total signal. This extracted noise component is compared with the original signal of a tone source in a subsequent amplification-calculation stage. The result of this comparison is then used to automatically adjust the volume of the acoustic useful signal output to the listener.
A problem with this technique is that the noise component cannot be completely extracted. As a result, components of the acoustic useful signal are contained in the extracted noise component. Therefore, the overall apparatus uses a noise level that is higher than the actual noise level as a basis to automatically adjust the volume. This causes the volume to be increased. This process is often referred to as xe2x80x9cgain chasexe2x80x9d which ends with the maximum possible volume level being set, causing the listener in the motor vehicle manually intervene in order to reduce the volume. To prevent xe2x80x9cgain chasexe2x80x9d U.S. Pat. No. 5,434,922 discloses considerable additional complexity.
Therefore, there is a need for a technique of automatically adjusting the volume in order to compensate for noise within the listening environment.
Briefly, according to the present invention, an apparatus includes a tone source that generates an electrical useful signal, which is adjustable by a control signal. An acoustic transducer receives the electrical useful signal and generates an acoustic signal indicative thereof. An acoustic receiver generates an electrical listening signal from the acoustic useful signal as well as from an acoustic noise signal at the listening location. An extractor extracts the useful signal component contained in the listening signal. A control device whose input is coupled to the acoustic receiver and to the extractor, and whose output is coupled to the tone source, controls the tone source as a function of the result of comparing the useful signal component contained in the listening signal with the listening signal itself. If, for example, a certain noise signal component is also contained in the extracted useful signal component, the result is the volume of the acoustic useful signal is reduced, which consequently leads to a lesser noise signal component in the extracted useful signal. However, this decrease of the acoustic useful signal level compared to the noise signal level causes the influence of the noise signal level on the volume to increase, and consequently increases the accuracy of the entire apparatus. Finally, the entire apparatus will swing into a stable value.
The present invention is also directed to an invention method that creates the acoustic useful signal from a electrical useful signal, and records an electrical listening signal determined by the acoustic useful signal and by an acoustic noise signal at the listening location. The listening signal is processed to extract the useful signal component contained in the listening signal. The useful signal component contained in the listening signal is compared with the listening signal itself, to generate a control signal that is used to adapt the electrical useful signal as a function of the result of the comparison step.
The extraction preferably is performed by adaptive filtering such that the useful signal component contained in the audible signal is extracted from the electrical audible signal by using the electrical useful signal taken from the tone source. Adaptive filters have the advantage of achieving high precision with relatively little complication. The adaptive filtering process preferably proceeds utilizing the least means square (LMS) method.
To control the volume, the ratio of the useful signal component contained in the audible signal to the audible signal itself is evaluated. To form this ratio the logarithms are taken of the levels of the useful signal component contained in the audible signal and of the audible signal itself, and these level logarithms are subtracted from one another. This corresponds to the formation of a ratio with subsequent formation of the logarithm, which can be more useful, for example, for adjusting the volume. Forming the two levels with subsequent subtraction has the advantage of being implemented easily, especially in digital circuit technology. As the level, one can use, for example, the level of the average absolute values or preferably the power level.
The entire apparatus is preferably designed at least partially in digital circuit technology. Similarly, the inventive method is implemented, for example, by an appropriately programmed digital signal processing device (e.g., a signal processor). The electric useful signal and electric listening signal are digital signals and/or digitized signals with a certain sampling rate. The extraction and/or the comparison is then performed at a reduced sampling rate. This measure can substantially reduce the entire calculation complication.
The present invention is delineated against previous solutions especially by requiring extremely little additional complication. Furthermore, the invention assures in principle that the volume will not automatically increase under any circumstances.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.